(1) Field of the Invention
The invention is related to a flexbeam unit for a multi-blade rotor of a rotary wing aircraft, said flexbeam unit comprising a plurality of flexbeam elements with the features of claim 1.
(2) Description of Related Art
A flexbeam unit is used in a so-called bearingless or a so-called hinge- and bearingless rotor system to connect rotor blades of a multi-blade rotor of a rotary wing aircraft to an associated rotor shaft of the aircraft. During operation, i.e. rotation of the multi-blade rotor, the flexbeam unit must withstand and transfer tremendous centrifugal forces that the rotor blades apply thereto, while permitting their flapping, pitch and lead-lag motions. Therefore, the flexbeam unit comprises special, in particular fiber reinforced composite material flexbeam elements that are flexible enough in torsion to allow twisting for blade movement without discrete bearings in the case of a bearingless or a hinge- and bearingless rotor system.
These flexbeam elements usually possess lead-lag-soft regions that permit motions of associated rotor blades in a bearingless or a hinge- and bearingless rotor system in the lead-lag direction. The lead-lag-soft regions thus constitute fictitious vertically oriented axes, so-called virtual lead-lag hinges, about which the rotor blades execute forward and backward lead-lag motions. Furthermore, these flexbeam elements realize flapwise-soft regions that enable flapping of the associated rotor blades in the vertical direction and, thus, constitute fictitious horizontally oriented axes, so-called virtual flapping hinges, about which the associated rotor blades execute upward and downward flapwise motions in a bearingless or a hinge- and bearingless rotor system. The distance between these virtual flapping hinges and the axis of the rotor shaft is referred to as the flapping hinge distance.
Moreover, only in a hinge- and bearingless rotor system, these flexbeam elements usually comprise torsion weak regions which enable low-force torsional motion of the flexbeam elements for inducing pitch angle adjustments of the rotor blades, and which allow limiting an associated length of the flexbeam elements. Such torsion weak regions must be resistant against lead-lag and flap shear forces and provide a required stiffness in lead-lag and flapping direction for dynamic reasons. Furthermore, such torsion weak regions should have a small cross section in order to reduce an associated drag of these regions.
More specifically, for controlling an associated torsion of the torsion weak regions of the flexbeam elements in order to set a current pitch or blade angle of the rotor blades, suitable control cuffs are associated with the flexbeam elements and, in particular, with the torsion weak regions. The control cuffs are connected to associated root ends of the rotor blades and associated flexbeam heads of the flexbeam elements at predetermined disconnecting points provided at the flexbeam heads of the flexbeam elements. An underlying radial position of these disconnecting points with respect to the rotor shaft of the multi-blade rotor is mainly defined by a given axial length of the control cuffs and the flexbeam elements, and is usually located at a comparatively high radial distance from the rotor shaft.
As the control cuffs cause a disturbance of an aerodynamic airflow at the multi-blade rotor, which disturbance increases with the radial distance of the disconnecting points from the rotor shaft, the given axial length of the control cuffs and their projected profile need to be reduced in order to reduce an associated aerodynamic drag. However, reducing the given axial length of the control cuffs inherently implies reducing an underlying axial length of the flexbeam elements, so that comparatively short and compact flexbeam elements can be provided which allow for a high torsion angle per length unit.
The document U.S. Pat. No. 4,352,631 describes a flexbeam element that is made of fiber compound material and that functions as a flapping hinge, a lead-lag hinge and a torsion hinge. This flexbeam element has a cruciform cross section and comprises at least two components arranged at an angle relative to each other. At least one component, which is made up of main plies comprising unidirectional fibers, extends outside the boundary of the other component on both sides of the other component, which is made up of bias plies comprising fibers that are oriented transversely relative to said unidirectional fibers. Slots are arranged in at least one of the components and these slots extend substantially towards an associated shearing center or rather towards a central shearing axis of the flexbeam element.
However, the cruciform cross section of this flexbeam element needs to be passed into flat rectangular cross sections at an associated flexbeam root and head for connection to the rotor shaft and an associated rotor blade, which makes manufacturing of this flexbeam element difficult. Furthermore, the cruciform cross section still provides for a comparatively high torsional stiffness and a peak stress in the unidirectional fibers is also comparatively high.
The document U.S. Pat. No. 5,358,381 describes a flexbeam element having a central part with radially extending flanges. Each flange comprises a bias pack that is embedded between two unidirectional belts, which in turn are surrounded on their outer surface by a thin cover. The bias pack is constructed of four plies of ±45° fiberglass material, the unidirectional belts are constructed of unidirectional fiberglass material and the thin cover consists of woven fiberglass material, i.e. of bias plies. All bias packs are intersected at an associated web defining the central part that is, thus, formed by these bias packs, while the unidirectional belts have a thickness that decreases towards said web, so that the unidirectional belts are not joined to said web. However, the bias packs that are interconnected at the web define notches at the central part. Plies with unidirectional fibers have been added in the notches at the central part in order to reduce a given corner peak stress in shear.
However, the plies with the unidirectional fibers are not appropriate to reduce shear stress occurring in the notches at the central part. Furthermore, the plies with the unidirectional fibers at the central part are not appropriate to avoid delamination at the central part. Moreover, as shear forces occurring at the flanges are low and as the flanges are subjected to the highest normal stresses, covering the plies with unidirectional fibers of the flanges with bias plies can be omitted and, thus, unnecessarily complicates manufacturing of the flexbeam element, as the bias plies are not appropriate for resistance to normal stresses and have lower strength, especially fatigue strength, as unidirectional fibers.
The document US 2006/0193727 A1 describes a flexbeam element that is made up mostly of composite fiber material. This flexbeam element has a cross section with a substantially symmetric, flattened shape that has approximately the contour of a horizontal, central section of a double cone, i.e. two cones having their base sides arranged at opposing sides of the cross section. Furthermore, the flexbeam element comprises pairs of packets with unidirectional reinforcement fiber plies that are respectively separated by slots, wherein U-shaped or loop-shaped layers with reinforcement fiber fabric plies are arranged.
However, a central part of the cross section of this flexbeam element is comparatively large and an underlying radius at an inner end of each slot is comparatively narrow. This is difficult to manufacture and leads to higher shear peak stresses at the inner, narrow radii. Furthermore, the cross section as a total is larger than necessary for a required stiffness in lead-lag and flapping direction and the overall triangular shape of the double cone is difficult to manufacture.
It should be noted that further or similar flexbeam elements are also described in the documents WO 2004/089745 A1, EP 2 246 256 A1, EP 1 431 176 A1, DE 196 20 427 C1, EP 0 323 857 A2, IT 8 322 677 D0, CA 2 320 606 A1, JP 61021894 A and WO 2004/085248 A1. However, all flexbeam elements described by the prior art documents cited above have central parts, which are considered to define the most sensible areas of the flexbeam elements, having comparatively large cross section areas, which could still be reduced, and/or central parts having only plies with unidirectional fibers in high loaded areas, where relatively high shear peak stresses occur during operation.
It is, therefore, an object of the present invention to provide a flexbeam unit for a multi-blade rotor of a rotary wing aircraft, said flexbeam unit comprising a plurality of flexbeam elements that are suitable to overcome the above described drawbacks of the prior art, and that are comparatively easy to manufacture with a reduced length and a compact structure that allows for a high torsion angle per length unit.